Solid oxide fuel cells (SOFCs) are energy conversion devices that are capable of efficiently converting chemical fuels directly to electrical power. They typically consist of a three-layer electrochemical cell, including a cathode, an electrolyte, and an anode. Oxygen molecules are catalytically reduced to oxide ions at the cathode, and the ions diffuse through the electrolyte to reach the anode. At the anode, a fuel (e.g., hydrogen or a syngas) reacts with the oxide ions, releasing electrons. Because the electrolyte is non-conducting, the electrons are forced to return to the cathode through an external circuit, where the derived energy is put to work.
A typical anode is a porous nickel cermet with yttria-stabilized zirconia (YSZ), in which nickel functions as both a catalyst for fuel oxidation and a conductor of electrons to the external circuitry. Fuel cells powered by the oxidation of hydrogen or hydrocarbon fuels rely on thermal H—H or C—H bond activation at the anode, and as a result, they typically operate at temperatures between about 700° C. and about 1,000° C. A common problem when using hydrocarbon fuels is the propensity of nickel-based anodes to suffer carbon deposition (or “coking”) due to pyrolysis of the fuel at the hot catalyst surface. The resulting carbon deposits form a barrier layer on the anode, preventing reaction of nickel with the fuel and reducing the conductivity of the anode. In fuel cells where the anode serves as the structural support of the cell, coking can also compromise the mechanical integrity of the fuel cell. Under unfavorable conditions, SOFC anodes can be deactivated by coking in as little as several hours. For this reason, most commercially-available SOFCs are designed to operate with hydrogen as the fuel.
Hydrocarbon fuels such as natural gas, propane, gasoline, kerosene and diesel are less expensive, more easily and safely stored, and more readily available than hydrogen. Synthetic methanol and plant-derived ethanol are also under consideration as fuels. However, the problem of coking often prevents the use of these fuels in most currently available SOFCs. A possible solution is to include a reformer device as a component of the fuel cell which can convert a hydrocarbon fuel into a mixture of carbon monoxide and hydrogen (i.e., a syngas) by catalytically reacting the hydrocarbon fuel with oxygen (partial oxidation reforming) or water (steam reforming).
Steam reforming produces carbon monoxide and hydrogen by catalysis of the following reaction:CnHm+nH2O→nCO+(m/2+n)H2 The process is highly endothermic, and consumes a considerable amount of energy which is typically supplied by external combustion or in situ partial oxidation (autothermal reforming) of the fuel.
In partial oxidation (POX) reforming, the fuel is partially oxidized with O2 over a catalyst to produce carbon monoxide and hydrogen. The reaction is exothermic, but at the cost of a lower yield of hydrogen:CnHm+(n/2)O2→nCO+(m/2)H2 
Autothermal reforming is a process in which both steam reforming and partial oxidation reforming reactions occur simultaneously. The energy released by the exothermic partial oxidation reaction drives the endothermic steam reforming reaction.
Because carbon monoxide can poison many reforming catalysts, the gas streams produced by any of the above reforming reactions usually are also subjected to a water gas shift reaction to convert the carbon monoxide into carbon dioxide:CO+H2O→CO2+H2 Any residual carbon monoxide (typically 1-2%) can be removed with a gas-separation membrane, and the hydrogen is then used to fuel the SOFC.
There have been several efforts to design “integrated” fuel reformer/fuel cell systems, some of which capture the waste heat and/or water generated by the fuel cell to drive endothermic fuel reforming reactions. However, even the most closely-associated fuel reformer/fuel cell pairings remain separate devices under the cover of a single module. The inclusion of reformer units and gas separation devices increases the cost and complexity of a fuel cell, and imposes additional downtime and maintenance costs. The added weight of the fuel reforming hardware is a further disadvantage in portable applications, such as power supplies for electric or hybrid vehicles, wilderness and battlefield electronics, and aircraft. Furthermore, although fuel reformers can function smoothly and efficiently under steady-state conditions, they are difficult to operate in an environment of intermittent and variable energy demand.
Fuel cells that can directly oxidize pure methanol have been developed, but at present, they are costly and relatively inefficient, competitive only with equally costly rechargeable lithium-ion batteries. Accordingly, there remains a need for fuel cells that can operate directly on unreformed hydrocarbon fuels without suffering from anode degradation due to coking.